Extrusion heads for continuous extrusion forming of continuous plastic elements having specific cross-sectional shapes are well known. Such extruded elements may include, for example, pipes, rods, moldings, tubings, and the like.
In a typical prior art extrusion system, solid pellets of the thermoplastic material to be used are fed into a progressive-screw extruder wherein the pellets are liquefied under high pressure and are injected into an extrusion head. Such injection may be made axially of the extrusion head, as is commonly done for extrusion of solid shapes and tubes, or transversely of the extrusion head, as is commonly done for coating of a core stock being passed axially through the head. Axial injection is known in the art as “inline,” and transverse injection is known in the art as “crosshead.” In cross-head injection, the molten extrudate enters the extrusion head at an angle, typically 90°, to the axis of the head. Crosshead extrusion is especially useful in applying coatings to rigid core forms such as rods, wire, and lumber.
Prior art extrusion heads are known to be used in forming longitudinal voids, inclusions, and stripes in extruded shapes such as rods, tubes, and core material coatings. For example, the colored insulative coating on electric wire such as bell wire typically includes a different colored stripe to aid in wire identification in a wire bundle.
A particularly demanding extruded form is tubing having a central passage and one or more smaller passages formed in the tubing wall. Such passages are known in the art as “lumens.” The lumens may be open passages or may be filled with different colors of the wall material or may be filled with entirely different material. For example, a medical catheter may require one or more open wall lumens and may also require a lumen filled with a radio-opaque material such as barium sulfate. The formation of such tubing and lumens requires a high level of precision in the extrusion dies and in the placement of die elements for forming the lumens.
In a prior art cross-head extrusion head for forming tubing having lumens, molten polymer is injected into the head and is shaped into an annular flow around a cylindrical mandrel. The flow is then squeezed along a conically tapered section of the mandrel and then along another, smaller diameter cylindrical section of the actual extrusion tip and die. An axial pipe extends through the mandrel and the die as an extrusion tip for forming and supporting the principal axial passage in the extruded tubing. Typically, air is injected through the central pipe to keep the tubing properly inflated as it is extruded. A linear pipe of smaller diameter, such as a hypodermic needle and referred to herein as a “lumen pipe,” is disposed in an off-axis passage formed through the mandrel parallel to the central pipe and extends to the extrusion end of the die. During tubing extrusion, air is injected through the lumen pipe to continuously form a void (lumen) in the tubing wall. If a filled lumen is desired instead, an appropriate material may be injected through the lumen pipe instead of air.
At least two significant problems are known in forming lumens in the prior art.
First, because the lumen pipes are disposed in bores in the mandrel, the positions of the pipes are fixed. This means that the position of the pipe within the die cannot be adjusted, although such adjustment may be highly desirable to compensate for errors in bore location or in the pipes themselves. Also, a user may desire to change the radial location of a lumen. Further, because the lumen pipes become part of the mandrel, their radial position is fixed with respect to the inner diameter of the extrusion. Both the mandrel and the die must be changed to change over to a different diameter extrusion.
Second, because the lumen pipes extend from the mandrel, they must necessarily intersect the conical portion thereof. It is known that flow disturbances around the pipes at their angular exit from the mandrel can remain in the wall of the extruded tubing and can be both cosmetically and functionally undesirable.
Third, because the diameters of the off-axis bores are fixed and selected for a given diameter lumen pipe, each mandrel is dedicated to extrusions having that size lumen. The diameter of a lumen cannot be changed without changing the mandrel. Thus, for a facility producing a large number of different extrusions, an equally large number of different mandrels is required.
What is needed in the art is a means for extruding a shape having at least one wall lumen wherein the lumen pipe may be accurately and adjustably positioned when an extrusion head is assembled.
What is further needed is a means whereby the position of the lumen within the wall may be readily adjusted as desired after assembly of an extrusion head.
What is further needed is a means whereby a single mandrel may be used for a plurality of lumen-forming extrusion dies.
What is further needed is a means for preventing or minimizing flow disturbances in the walls of lumen-containing shapes.
What is further needed is a means for changing a lumen pipe without having to change either the die or the mandrel.
It is a principal object of the present invention to provide apparatus for extruding lumen-containing shapes wherein the location of a lumen within the shape may be accurately established prior to beginning of extrusion and may be readily adjusted as desired during extrusion.
It is a further object of the present invention to provide apparatus for extruding lumen-containing shapes wherein a plurality of such shapes may be generated without requiring a change in the extrusion mandrel.